ELECTROCHEMICAL AND ULTRAHIGH-VACUUM CHARACTERIZATION OF ULTRATHIN CU FILMS ON PT(111)

Submonolayer to multilayer coverages of Cu electrodeposited and vapor-deposited onto a well-characterized Pt(111) surface were studied in both ultrahigh vacuum (UHV) and electrochemical environments by use of a combined UHV-electrochemical transfer apparatus. Linear sweep voltammograms obtained in 0.2 M HClO4 for both types of surfaces displayed very similar electrochemical stripping profiles. The potential difference between the multilayer and the underpotential deposited (UPD) monolayer stripping peaks is approximately 0.43 V, which is equivalent to an absolute energy difference of 19.8 kcal/mol. In addition, these voltammetric results demonstrated that simple site blocking for hydrogen chemisorption on Pt(111) by both electrodeposited and vapor-deposited Cu occurs linearly with chemisorption being essentially eliminated at theta-Cu almost-equal-to 1.0 ML, much similar to analogous studies performed in a UHV environment. Although the charge measured for the monolayer Cu stripping peak in both cases is lower than the theoretically expected value for a Cu monolayer in registry with the Pt(111) substrate surface, UHV studies of Cu overlayers on Pt(111) based on low-energy diffraction (LEED) analysis, Auger electron spectroscopy (AES) analysis, work function measurements, and photoemission studies have shown that a complete Cu monolayer is formed prior to the formation of Cu multilayers. To account for the discrepancy between these two values, we believe that the UPD Cu monolayers in the electrochemical environment near their stripping potentials are partially charged as a consequence of transfer of electronic charge from the metal adlayer to the Pt(111) substrate.